Abstract

Entropy and unavoidable irreversibility place a limit on the efficiency of photochemicalsolar energy conversion which is substantially lower than that placed by the first law of thermodynamics alone. Shockley and Queisser’s ’’detailed balance limit’’ on the efficiency of p‐n‐junction photovoltaic devices is a special case of this general thermodynamic limit on the efficiency of all quantum‐utilizing solar energy converters. For a single photochemical system operating at 20 °C in sunlight not attenuated by the atmosphere, this efficiency cannot exceed 29%. Under the same conditions, the efficiency of a solar converter composed of two photochemical systems can reach 41%.